In a hydraulic control system for hydraulic machines such as hydraulic excavators, a valve device having a spool type directional control valve built therein is used as means for switching over passages for a hydraulic fluid from a hydraulic pump and transmitting the hydraulic fluid to a target actuator. Such a valve device includes a spool slidably fitted to a spool bore formed in a valve body, and controls the flow rate and the flowing direction of the hydraulic fluid supplied from the hydraulic pump to the actuator when the spool is operated.
In the above valve device, an appropriate clearance is formed between the spool and the bore of the valve body so that the spool can slide in the bore. For the presence of the clearance, there occurs internal leakage at passage portions around the spool, particularly at a port portion communicating with the actuator. Where the actuator is a hydraulic cylinder for holding a high load, the internal leakage appears as a reduction in the load holding force. Specifically, if the load pressure of the hydraulic cylinder is enclosed at an actuator port portion around the spool, the hydraulic fluid internally leaks from an outer peripheral of the spool to a reservoir port, causing the hydraulic cylinder to move a little by a little with the elapse of time.
To cope with such a problem, a hydraulic control system including a non-leak valve built in a valve device is proposed in JP, Y, 7-47604.
In the valve device of the hydraulic control system proposed, a non-leak valve is disposed in an actuator passage formed in a valve body to interconnect an external actuator port connected to a hydraulic cylinder and an actuator port around the spool (i.e., an internal actuator port). The non-leak valve comprises a main valve section including a seat valve positioned in the actuator passage, and a pilot control section including a pilot poppet valve for controlling opening/closing of the seat valve of the main valve section. When the spool is in the neutral position, the seat valve is closed to minimize the amount of internal leakage. When the spool is operated, the pilot poppet valve of the pilot control section is opened in interlock with the movement of the spool to make open the seat valve of the main valve section.
Generally, an overload relief valve is incorporated in a valve device for use with a hydraulic cylinder for holding a high load so that when the hydraulic cylinder is subject to an overload, the overload relief valve is opened to release the hydraulic fluid under high pressure to a reservoir, thereby preventing the hydraulic cylinder from being damaged. The valve device described in JP, Y, 7-47604 also includes an overload relief valve disposed therein separately from the non-leak valve. Specifically, the non-leak valve is arranged in the valve body parallel to the spool, and the overload relief valve for preventing the actuator from experiencing an overload is arranged on the outer side of the non-leak valve parallel to the non-leak valve and the spool. Furthermore, the overload relief valve is usually added with a make-up function of replenishing the hydraulic fluid from the reservoir when the hydraulic fluid supplied to the actuator is subject to negative pressure.
In addition, JP, A, 3-249411 describes a load fall preventing valve device which is separate from a valve device including a spool built therein, and in which is built a non-leak valve having both a load holding function and an overload relief function.